Various types of abrasive cutting tools are known in the art, and in general their fabrication depends in large part upon their intended application and upon the abrasive employed.
For example, in general grinding applications, where it is desired to remove stock from a workpiece, a cutting tool such as a grinding wheel may be mounted for rotational movement upon a machine spindle, and abrading is then achieved by bringing the workpiece into contact with the abrasive-containing periphery of the rotating wheel, or vice versa. Known abrasive grinding wheels are typically fabricated by compounding abrasive material with a binder, (along with various additives and coatings where, for particular applications these are required) and this compound is then cast or molded with heat and/or pressure being applied to bring about bonding of the compounded materials by setting of the binder, as by sintering or curing, or the like. Often such grinding wheels are fabricated of vitreous materials such as glass or ceramic frits and the like. A characteristic common to grinding wheels is that they are typically of homogenous composition, i.e., their abrasive(s) are dispersed throughout the wheel, and thus they may be used for extended periods until they become worn down to the attachment hub. The abrading surface of a grinding wheel typically requires occasional dressing back to the required grinding profile, and this is usually achieved by the application of a dressing tool having a hardness greater than that of the abrasive material of the wheel.
Because of their homogenous dispersion of abrasives, grinding wheels generally offer desirable economies of fabrication and operation and, as noted, have considerable service life with periodic dressing. Such economies, however, are obviated where, due to the hard nature of the material to be abraded or because an extremely fine surface finish is to be produced, it is necessary to compound such wheels with more exotic abrasive materials, such as diamonds or diamond-containing abrasives. Also, where frequent close-tolerance redressing to a particular profile is necessary, the labor and time required by the dressing operation alone with the concomitant machine "downtime" often obviates any other factors of economy.
Therefore it is often desirable to provide abrasive cutting tools which have abrasive-faced cutting surfaces, such as the diamond-faced drill disclosed by Taylor in U.S. Pat. No. 2,014,955 which has a number of diamonds embedded into the face of a molded, pressed and sintered tool. A disadvantage posed in practicing the method as taught by Taylor concerns the manual insertion of individual diamonds into a curved mold in which a thin layer of a plastic mixture has first been spread. As taught by Taylor, when employing diamonds large enough to be handled separately, these diamonds are pushed into and through this pasty plastic mixture layer until they come into contact with the mold surface in order to assure that the cutting point of each diamond will lie in the curved surface defined by the mold's cross-sectional curvative (i.e., at the cutting face), and so that no diamond will be above or below the zone of contact between the nose of the drill and the material being drilled. As further taught by Taylor, when employing diamonds too small to be handled separately and positioned in the thin plastic layer with tweezers, then these small diamonds may be poured onto the plastic layer into which the diamonds will sink under gravity to an extent which will provide a single layer of diamonds in the finished drill.
It becomes evident in practicing the method taught by Taylor for fabricating diamond-faced tools that such method is both labor intensive and imprecise in that the larger diamonds must be manually placed in the mold where their final orientation is dependent upon the retentive properties of the thin plastic matrix layer and subject to undesirable displacement during subsequent charging of the mold. When pouring smaller diamonds into the plastic mixture layer, it is critical that the consistency of the plastic mixture be such as to permit the diamonds to sink therethrough readily while thereafter retaining the diamonds therein when the mold is inverted to remove the diamonds which are not in contact with the paste layer. And in the latter case it becomes difficult to control or determine the proper placement of the diamond layer at the cutting face when the mold's cross-section is curved because the diamonds will tend to gravitate, or settle out, to the lowest portion of the mold and thus leave the upper surfaces of the cutting face unclad.
James, in the U.S. Pat. No. 3,625,666, teaches a method of forming metal-coated diamond abrasive wheels whereby diamond particles, having first been coated with nickel, are charged, in a fluid epoxy resin mixture, into a mold, and where settling out of the diamonds is prevented and the distribution of the particles in the fluid resin composition is controlled by applying a magnetic or electrostatic field across the mold. The direction of the lines of the applied field force is chosen to be normal to the eventual working face of the wheel so that the elongated diamond particles tend to align themselves axially along the lines of applied force. The applied field is maintained until the epoxy resin hardens. James further teaches the provision of asperities (e.g., ribs, raised screw threads, knurls, cones) in opposed faces of the mold cavity whereby, the applied field may be concentrated more strongly along certain lines therebetween so as to cause the diamond particles to arrange and align themselves between pairs of opposing asperities having opposite polarity.
Diamond wheels fabricated according to James offer the advantage of better distribution of the diamonds through the expoxy binder matrix as well as controlled orientation of their cutting faces, but present also the disadvantage that because the diamonds are distributed throughout the body of the tool rather than at its working face, this particular fabrication method is overly costly as it necessitates the use of more diamond material than in tools where the diamond abrasive is present only at the working tool face.
James further suggests forming the diamond-containing abrasive body as a separate element which, after forming, may be then applied to an appropriate base. Molding of resin-bonded diamond abrasive bodies is taught also by U.S. Pat. No. 4,246,004 to Busch et al., and the method therein proposed is applicable as well to the fabrication of abrasive bodies containing other abrasive materials than diamonds, so long as the abrasive particles employed are "needle shaped." The abrasive body disclosed by Busch et al., is a curved segment, a plurality of which may be bonded to a hub to provide a cup grinding wheel. As in the method of James, Busch et al., teach coating the "needle shaped" abrasive particles and during molding of the segment a magnetic field is induced directionally across the mold whereby the "needle shaped" abrasive particles tend to orientate with the long axes parallel with the lines of applied field force so that in the assembled cup grinding wheel their cutting faces are normal to the working face of the tool. But as with James' method, abrasive bodies fabricated in accordance with the method of Busch et al. necessitates using more diamond material than in tools where diamond abrasive is present only at the tool face, and further, the latter method requires that "needle shaped" abrasive particles be selected from batches of abrasive material, which further complicates fabrication and adds undesirably to the cost of fabricating abrasive tools.
Phaal, in U.S. Pat. No. 4,203,732 suggests a method for molding an abrasive grinding wheel rim having "needle shaped" abrasive particles whereby a mixture of abrasive particles and a resin bonding matrix is made to flow through the constricted passages of a mold thus causing the abrasive particles to orient with their long axes substantially in the direction of flow, whereafter the mixture is allowed to set. While the orientation method proposed by Phaal does not require using nickel-coated abrasive particles with an external energy field, it still suffers from the disadvantages that tools produced thereby will be more costly than tools having abrasive material such as diamonds present only at the working tool face, and suffers as well from the necessity of additional fabricating steps, and cost, of attaching the formed abrasive body to a suitable carrier in order to provide a usable tool.
Thus, from an economic standpoint alone, it is desirable, especially for profile machinery, to provide a tool which may be fabricated with an abrasive facing only at its working face(s), the remainder of the tool being constructed from easily formed and less expensive materials.
In certain abrasive machining operations, such as in the fabrication of graphite electrodes for use in electrochemical machining, a profiled form must be created which corresponds to the configuration of the object to be formed by the electrochemical machining operation such as in the case of a die cavity. It is known, for "total form machining", to employ an abrasive electrode form having a shape which is the reverse or mirror image of an electrode to be produced and which is constructed of abrasive particles held in a plastic matrix whereby an electrode may be abraded from electrode material (e.g., graphite) on movement of the electrode form with respect to the electrode material, as exemplified by U.S. Pat. Nos. 3,663,786 and 3,948,620.